A process for producing acrylonitrile or methacrylonitrile by bringing propylene, alternatively isobutene or tertiary butanol, into vapor-phase contact with a catalyst in the presence of ammonia and molecular oxygen is widely known as ammoxidation of an olefin and is practiced on an industrial scale at present. In this process, the molar ratio of ammonia to propylene in a reaction gas to be fed is usually set at 1.1 or more in order to increase the yield of the unsaturated nitrile, but the gas exhausted from a reactor contains a large amount of unreacted ammonia, owing to a disturbance of stoichiometric balance. A technique for treating the unreacted ammonia is an important problem from the viewpoint of the rationalization of the process, the prevention of environment pollution, etc., and various techniques have been developed in order to solve such a problem. For example, JP-B-49-25560, JP-B-51-49146, JP-A-52-4500, etc. disclose that the unreacted ammonia is separated and recovered as ammonium sulfate by the use of sulfuric acid. JP-B-51-3131 discloses that after the recovery of ammonia from the produced ammonium sulfate, the residual solution is incinerated. Deepwell injection of this waste solution is also carried out but the above-mentioned method associated with the use of sulfuric acid is not a preferable technique from the viewpoint of environmental pollution because SOx is generated when incinerating the produced ammonium sulfate.
On the other hand, as a method for treating the unreacted ammonia, JP-B-40-6451 and JP-A-52-32893 disclose the employment of an organic acid such as acetic acid, citric acid or an organic polybasic acid instead of sulfuric acid, and JP-B-45-33538 discloses the employment of carbonic acid gas. These methods permit treatment of the unreacted ammonia in the gas exhausted from the reactor, but they require the improvement and increase of facilities, etc., resulting in a complicated procedure, and moreover they are not economically advantageous.
As another method, i.e., a method in which substantially no unreacted ammonia is exhausted from the reactor, JP-A-7-53494, JP-A-7-126237 and JP-A-51-16615 disclose a method of supplying an organic compound to the reactor to reduce or substantially eliminate the unreacted ammonia. However, this method permits reduction or curtailment of the unreacted ammonia in the gas exhausted from the reactor but it requires further provision of other facilities in the reactor, resulting in complicated operations.
As yet another method for reducing or curtailing the unreacted ammonia in the gas exhausted from the reactor, JP-B-53-18014 discloses a method of carrying out the reaction while adjusting the molar ratio of ammonia to propylene in a reaction gas to be fed to a value near 1.0 by the use of a catalyst composed mainly of iron and antimony. It can be seen, however, from the results of analysis of the reaction product which are disclosed in this reference, that no acrylic acid was produced as a by-product.
JP-A-48-72122 discloses a method of regulating the ammonia concentration in a reaction gas to 0.8 vol % or less by adjusting the molar ratio of ammonia to propylene to 0.95-1.1 by the use of a catalyst containing as essential constituents molybdenum, bismuth, iron, cobalt, nickel, phosphorus and sodium. However, also in this method, the employment of sulfuric acid is indispensable for treating the unreacted ammonia.
JP-B-45-2371, JP-B-62-46538, JP-A-7-82228, etc. describe analytical values for acrylic acid in addition to the yield of acrylonitrile. JP-A-55-13187 describes analytical values for acrylic acid and unreacted ammonia in addition to the yield of acrylonitrile. However, none of these references mention the molar ratio of an organic acid to unreacted ammonia in an ammoxidation product gas and disclose that the unreacted ammonia is reacted with the organic acid (e.g. acrylic acid or acetic acid) produced by the ammoxidation, to be fixed as ammonium salt of the organic acid.